The present invention relates to a composite separator. The present invention also relates to a lithium-ion battery having such composite separator, and a method for producing such composite separator.
As used herein, the terms “lithium-ion battery”, “rechargeable lithium-ion battery”, and “secondary lithium-ion battery” are used synonymously. These terms also encompass the terms “lithium battery”, “lithium-ion accumulator”, and “lithium-ion cell”, and also all lithium or alloy batteries, including in particular lithium-sulfur, lithium air or alloy systems. Therefore, the term “lithium-ion battery” is used as a collective term for the aforementioned terms which are customary known in the art. It refers to both rechargeable batteries (secondary batteries) and non-chargeable batteries (primary batteries). In particular, as used herein, a “battery” within the meaning of the present invention also encompasses an individual or single “electrochemical cell”.
Generally, as known in the art, the mode of action of a lithium-ion battery can be described as follows: the electrical energy is stored in lithium ions (at the negative electrode) and transition-metal oxides (at the positive electrode) in a chemical process with a change of material. Here, the lithium ions in the ionized form (Li+) can migrate back and forth between the two electrodes through an electrolyte, which contains usually lithium hexafluorophosphate (LiPF6) as the lithium-conducting salt. In contrast to the lithium ions, the transition-metal ions present at the cathode are stationary.
This flow of lithium ions is necessary in order to compensate the external flow of electric current during charging and discharging, so that the electrodes themselves remain electrically neutral. During discharging, the effective lithium atoms (or the negative active material containing the lithium ions) at the negative electrode each release an electron, which flows via the external current circuit (load) to the positive electrode. At the same time, the same number of lithium ions migrates through the electrolyte from the negative electrode to the positive electrode. At the positive electrode, however, the lithium ions do not take up the electron again, but instead the transition-metal ions present there take up the electrons. Depending on the type of battery, these ions may be cobalt, nickel, manganese or iron ions, etc. The lithium thus continues to be in ionized form (Li+) at the positive electrode in the discharged state of the cell.
Lithium-ion batteries are protected with gastight sealing, and so in regular operations none of the ingredients can emerge or enter. If the casing is damaged mechanically, as it may occur for example, in the event of an accident involving an electric motor vehicle, contents may emerge in vapor, gas or liquid form. Emerging in gas form, primarily, are vaporized electrolyte (an explosion risk) and electrolyte decomposition products such as methane, ethane, hydrogen, propane and butane, and aldehydes. Emerging in liquid form, the liquid electrolyte consisting of solvents and conducting salt. The solvents are generally flammable and are toxic. In contact with moisture, the conducting salt LiPF6 can form hydrogen fluoride (HF) which is highly toxic and can be an irritant to the respiratory tract.
It is an object of the present invention to provide a lithium-ion battery with enhanced safety.
This and other objects of the invention are achieved by means of a composite separator in accordance with one or more aspects of the disclosure.
The following definitions apply, where applicable, to all aspects of the disclosure:
Lithium-Ion Battery
As used herein, the term “lithium-ion battery” has the meaning as defined above. In particular, the term also includes an individual or single “electrochemical cell”. Preferably, in a “battery”, two or more electrochemical cells of this kind are connected, either in series (that is, one after another) or in parallel.
Electrodes
The electrochemical cell of the invention has at least two electrodes, i.e., a positive electrode (cathode) and a negative electrode (anode).
These two electrodes each have at least one active material. This material is capable of accepting or releasing lithium ions and at the same time takes up or releases electrons.
As used herein, the term “positive electrode” refers to the electrode which when the battery is connected to a load, such as to an electric motor, is capable of accepting electrons. In this nomenclature, it represents the cathode.
As used herein, the term “negative electrode” refers to the electrode which in operation is capable of releasing electrons. In this nomenclature, it represents the anode.
The electrodes include inorganic material or inorganic compounds or substances which can be used for or in or on an electrode or as an electrode. Under the operating conditions of the lithium-ion battery, on the basis of their chemical nature, these compounds or substances can take up (intercalate) lithium ions or metallic lithium and also release them. In the present description, a material of this kind is referred to as an “active cathode material” or “active anode material”, respectively, or, generally, as “active material” or “active electrode material”. For use in an electrochemical cell or battery, this active material is preferably applied to a support, preferably to a metallic support, preferably using aluminum for the cathode and copper for the anode, respectively. This support is also referred to as a “collector” or a “current collector” or a “collector foil.”
Cathode (The Positive Electrode)
As for selecting the active material for the positive electrode (also referred to as the active cathode material), it is possible to use any active materials which are known in the art. These include, for example, LiCoO2 (LCO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), high-energy NCM (HE-NCM), lithium-iron phosphate, or Li-manganese spinel (LiMn2O4). According to one aspect of the invention, any suitable active material known in the art can be used for the cathode (the positive electrode).
In one preferred embodiment, the active cathode material may be a material selected from the group consisting of a lithium transition-metal oxide (also referred to as the lithium metal oxide), layered oxides, spinels, olivine compounds, silicate compounds, and mixtures thereof. Such active cathode materials are described for example in Bo Xu et al. “Recent Progress in Cathode Materials Research for Advanced Lithium Ion Batteries”, Materials Science and Engineering, R 73 (2012) 51-65. Preferably, the active cathode material is HE-NCM. Layered oxides and HE-NCM are also described in U.S. Pat. Nos. 6,677,082, 6,680,143 and 7,205,072 of the Argonne National Laboratory.
Examples of olivine compounds are lithium phosphates of empirical formula LiXPO4 where X=Mn, Fe, Co or Ni, or combinations thereof.
Examples of lithium transition-metal oxide, spinel compounds, and layered transition-metal oxides are lithium manganate, preferably LiMn2O4, lithium cobaltate, preferably LiCoO2, lithium nickelate, preferably LiNiO2, or mixtures of two or more of these oxides, or their mixed oxides thereof.
The active material may also contain mixtures of two or more of the substances described herein.
To increase the electrical conductivity, further compounds are included in the active material, preferably carbon-containing compounds, or carbon, preferably in the form of conductive carbon black or graphite. The carbon may also be introduced in the form of carbon nanotubes or graphene. Such additions are preferably in an amount of from 0.1 to 6 wt %, more preferably, from 1 to 3 wt %, based on the positive electrode's composition (excluding solvent) applied to the support.
Anode (The Negative Electrode)
As for selecting the active material for the negative electrode (also referred to as the active anode material), it is possible to use any active materials which are known in the art. According to one aspect of the invention, any suitable active material known in the art can be used for the anode (the negative electrode).
The active anode material can be selected from the group consisting of lithium metal oxides, such aslithium titanium oxide, metal oxides, (e.g., Fe2O3, ZnO, ZnFe2O4), carbon-containing materials, such as graphite (e.g., synthetic graphite, natural graphite), graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerenes, mixtures of silicon and carbon, silicon, tin, metallic lithium and materials which can be alloyed with lithium, and mixtures thereof. It is also possible to use niobium pentoxide, tin alloys, titanium dioxide, tin dioxide, and silicon as the active material for the anode (the negative electrode).
In one embodiment, the active anode material is a material which can be alloyed with lithium. This material may be metallic lithium, a lithium alloy, or an unlithiated or partially lithiated precursor thereof, from which a lithium alloy is produced on formation. Preferred materials which can be alloyed with lithium are lithium alloys selected from the group consisting of silicon-based, tin-based, and antimony-based alloys. Such alloys are described for example in the review article by W. J. Zhang, Journal of Power Sources, 196 (2011) 13-24.
Electrode Binder
The materials used for the positive or negative electrode, for example the active materials, are held together by one or more binders, which hold these materials on the electrode and/or on the collector.
The binders can be selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polyethylene oxide (PEO), polytetrafluoroethylene, polyacrylate, styrene-butadiene rubber, and carboxymethylcellulose (CMC), and mixtures and copolymers thereof. The styrene-butadiene rubber and optionally the carboxymethylcellulose and/or the further binders, such as PVdF, are preferably present in an amount of 0.5-8 wt %, based on the total amount of the active material used in the positive or negative electrode.
Separator
The electrochemical cell of the invention has a material which separates the positive electrode and the negative electrode from one another. This material is permeable to lithium ions, i.e., conducts lithium ions, but is a nonconductor for electrons. Materials of this kind used in lithium-ion batteries are also referred to as separators.
According to the invention, polymers are used as separators. In one embodiment, the polymers are selected from the group consisting of: polyesters, preferably polyethylene terephthalate; polyolefin, preferably polyethylene, polypropylene; polyacrylonitrile; polyvinylidene fluoride; polyvinylidene-hexafluoropropylene; polyetherimide; polyimide, polyamide, polyethers; polyetherketone, or mixtures thereof. The separator has porosity, and so that it is permeable to lithium ions. In a preferred embodiment, the separator consists of at least one polymer.
Electrolyte
As used herein, the term “electrolyte” refers to a liquid in which a conducting lithium salt is in solution. The liquid is preferably a solvent for the conducting salt. In that case the conducting lithium salt is preferably in a dissociated form.
Preferably, the solvents are chemically and electrochemically inert. Examples of suitable solvents include preferably organic solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolanes, 2-methyltetrahydrofuran, and 1,3-dioxolane. Preferably, organic carbonates are used as the solvent.
In one aspect of the invention, ionic liquids can also be used as solvents. The ionic liquids contain exclusively ions. Examples of cations include those which can be in alkylated form, such as imidazolium, pyridinium, pyrrolidinium, guanidinium, uronium, thiuronium, piperidinium, morpholinium, sulfonium, ammonium, and phosphonium cations. Examples of anions which can be used include halide, tetrafluoroborate, trifluoroacetate, triflate, hexafluorophosphate, phosphinate, and tosylate anions.
Exemplary ionic liquids include the following: N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide, N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide, N-butyl-N-trimethylammonium bis(trifluoromethylsulfonyl)imide, triethylsulfonium bis(trifluoromethylsulfonyl)imide, and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide.
Preference is given to using two or more of the liquids described above. Preferred conducting salts are lithium salts which have inert anions and which are preferably nontoxic. Suitable lithium salts are preferably lithium hexafluorophosphate (LiPF6), or lithium tetrafluoroborate (LiBF4), and mixtures of one or more of these salts. In one embodiment, the separator here is wetted or impregnated with the lithium salt electrolyte mixture.
Various objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying examples.
In one aspect of the disclosure, the present invention is directed to a composite separator.
The composite separator of the invention includes a polymer membrane, a binder, a solid inorganic lithium-ion conductor, and a liquid electrolyte, the solid inorganic lithium ion conductor in the composite separator being present in a higher volume fraction and weight fraction than the liquid electrolyte.
In one embodiment, the solid inorganic lithium-ion conductors include crystalline, composite, and amorphous inorganic solid lithium-ion conductors. The crystalline lithium-ion conductors include, in particular, perovskite-type lithium lanthanum titanates, NASICON-type, LiSICON-type and thio-lisicon-type Li-ion conductors, and also garnet-type Li-ion-conducting oxides. The composite lithium-ion conductors include, in particular, materials which contain oxides and mesoporous oxides. Solid inorganic lithium-ion conductors of this kind are described for example in the review article by Philippe Knauth “Inorganic Solid Li Ion Conductors: An Overview”, Solid State Ionics, Volume 180, Issues 14-16, 25 Jun. 2009, pages 911-916. Also included in accordance with the invention are all solid lithium-ion conductors which are described in Cao C, Li Z-B, Wang X-L, Zhao X-B and Han W-Q (2014) “Recent Advances in Inorganic Solid Electrolytes for Lithium Batteries”, Frontiers in Energy Research, 2:25. Also included, in particular, in accordance with the invention are the garnets described in EP 1723080 B1.
The composite separator of the invention therefore has a composition which is predominantly a solid inorganic lithium-ion conductor that is employed as an inorganic solid-state electrolyte. Also present, as an auxiliary electrolyte, is a liquid electrolyte, in a lower weight fraction and volume fraction.
The inventors have recognized that by including the solid inorganic lithium ion conductor in the composite separator according to the present invention, it is possible to reduce the amount of liquid electrolyte in the composite separator. As a result, it is possible to significantly reduce the total amount of liquid electrolyte in a lithium-ion battery containing the composite separator. In this way, both the amount of solvents and the amount of conducting salt, especially LiPF6, can be lowered, hence making it possible to reduce not only the risk of ignition of emergent liquids or gases but also reduce the health hazards posed by the production of hydrogen fluoride (HF) from an reaction of LiPF6 with moisture.
In one preferred embodiment of the invention, the composite separator has interconnected pores which contain the solid inorganic lithium-ion conductor and the liquid electrolyte. By arranging the solid inorganic lithium-ion conductor and the liquid electrolyte in interconnected pores, it is possible to lower the contact resistance between the particles of the solid inorganic lithium-ion conductor.
In one preferred embodiment of the invention, the composite separator, based on the volume without the liquid electrolyte, possesses a porosity of 10% to 60% and the porosity is filled out with the liquid electrolyte to an extent of more than 90%, more preferably, more than 95%. Most preferably, it is completely filled out by the liquid electrolyte. By filling out the porosity with the liquid electrolyte to the most complete extent possible, it is possible to significantly improve the contact resistance between the particles of the solid inorganic lithium-ion conductor.
In one preferred embodiment of the invention, the polymer of the polymer membrane is selected from the group consisting of polyolefin, polyester, polyamide, polyimide, aramid, and mixtures thereof.
In one preferred embodiment, the solid inorganic lithium-ion conductor consists of particles, and the particle size D50 of the particles is more than 0.05 μm to 5 μm, preferably from 0.1 μm to 2 μm. The measurement values in this context are determined by scanning electron microscopy (SEM). A measurement technique of this kind is described for example in U.S. Pat. No. 5,872,358.
In one preferred embodiment, the solid inorganic lithium-ion conductor is present at 10 to 50 wt %, preferably from 20 to 40 wt %, in the composite separator in relation to the polymer membrane.
In one preferred embodiment, the solid inorganic lithium-ion conductor possesses a lithium-ion conductivity of at least 10−5 S/cm at room temperature (20° C.). The measurement values in this context are determined by the GITT (Galvanostatic Intermittent Titration Technique), as described for example in W. Weppner and R. A. Huggins, J. Electrochem. Soc., 124 1569-1578 (1977).
In one preferred embodiment, the solid inorganic lithium-ion conductor is selected from the group consisting of Perovskite, glass formers, Garnet, and mixtures thereof. Especially preferred are the Garnets described by EP 1723080 B1, on account of their particular chemical and electrochemical stability in the 3-5 V potential range of the cathode (positive electrode).
In one preferred embodiment, the binder is selected from the group consisting of polyvinylidene fluoride, copolymer of polyvinylidene fluoride and hexafluoropropylene, copolymer of styrene and butadiene, cellulose, cellulose derivatives and mixtures thereof.
In one preferred embodiment, the liquid electrolyte contains organic carbonates and a conducting salt, preferably LiPF6 or LiPF4.
The thickness of the composite separator is generally 1 μm to 250 μm, preferably 20 μm to 100 μm. The measurement values in this context are determined by optical methods, as specified in U.S. Pat. No. 4,008,523.
In another aspect of the disclosure, the present invention is directed to a lithium-ion battery containing electrodes, a separator, and an electrolyte, where the separator is a composite separator according to the present invention.
In another aspect of the disclosure, the present invention is directed to a method for producing the composite separator of the invention. The method includes the following steps: forming a suspension from a solid inorganic lithium-ion conductor and from a binder in solution with a solvent; providing a porous polymer membrane and filling up the porosity of the polymer membrane with the suspension; stripping off the solvent under reduced pressure and/or elevated temperature; and filling up the porosity with a liquid electrolyte.
The filling-up of the porosity may in each case be carried out by impregnation, optionally supported by reduced pressure and/or heat treatment.
The lithium-ion battery of the invention is suitable both for fixed and for portable applications. On account of the reduction in the amount of liquid electrolyte included, together with the reduced hazards to drivers/passengers, the lithium-ion battery of the invention is particularly suitable for use in motor vehicle applications.